The present invention relates to fluorinated ethylene-propylene copolymer powders having improved adhesion to substrates and a method of producing the improved powders. More particularly, the invention is directed to the irradiation of melt-processible fluorinated ethylene-propylene copolymer powders to improve the adhesion of the powders to metal substrates.
Fluorinated ethylene-propylene copolymers (generally referred to as FEP resins) are copolymers of tetrafluoroethylene and hexafluoropropylene and have properties similar to polytetrafluoroethylene resin. The repeating structure of the molecule may be represented as follows: EQU [--CF.sub.2 --CF.sub.2 --CF.sub.2 --CF(CF.sub.3)--].sub.n
Fluorinated ethylene-propylene copolymers have a number of excellent chemical and physical properties, including frictional, release and electrical properties and chemical resistance. These properties have resulted in the use of FEP resins as surface coatings on a variety of substrates, particularly metal substrates such as cooking utensils and the like. On the other hand, these same properties combine to make it difficult to adhere the copolymers to substrates. As a result, under dynamic conditions, the durability of the coated products is very poor.
Several approaches have been employed in attempts to improve the adhesion of FEP to substrates. For example, a film of FEP may be prepared in a form such that it may be expanded, placed over the substrate, and then subjected to heat to cause plastic memory shrink (heat shrinking). Using this technique, the mechanical strength of the polymer is exploited.
A second approach is to "etch" a sheet of FEP resin with an alkali metal organic compound, such as sodium naphthalene, sodium toluene, butyl lithium, or a variety of other compounds which will remove fluorine from the surface of the copolymer sheet. As a result, the "etched" surface has a much greater affinity for adhesives or for the substrate itself at elevated temperatures. Unfortunately, this method is time consuming, unsuitable for elaborate shapes, and can result in undesired modifications of the desirable properties of the FEP resin.
In any event, the two techniques cited above for improving coating durability are not applicable where the coating is to be applied in powder form. Thus, where it is desired to achieve a coating film thickness below 30 mils, the preferred method of coating is by powder application. A variety of powder application techniques are available, including particularly electrostatic spray, fluidized bed, electrostatic fluidized bed, and flocking.
Efforts to improve the adhesion of FEP powder to substrates have concentrated on preparation of the substrate surface and the utilization of primers. While these approaches have effected improvements in adhesion, satisfactory adhesion is still not obtained in many cases. These methods also necessarily involve extra manufacturing steps.
Furthermore, the use of primers is often not practical or compatible with powder application. Thus, many parts or products are coated by powder application simply because even application is difficult using a liquid dispersion of the FEP resin. Since primers are usually liquids, their use is also difficult in such cases.